Perl's comma statement separator has really low precedence,
which leads to code that looks like it's using the comma list element separator not actually doing so.
Conway suggests that the statement separator not be used in order to prevent this situation.

The confusion that the statement separator causes is primarily due to the assignment operators having higher precedence.

For example,
trying to combine two arrays into another like this won't work:

@x = @y, @z;

because it is equivalent to

@x = @y;
@z;

Conversely, there are the built-in functions, like print, that normally force the rest of the statement into list context, but don't when called like a subroutine.